$ \left(\dfrac{64}{81}\right)^{-\frac{1}{2}}$
Solution: $= \left(\dfrac{81}{64}\right)^{\frac{1}{2}}$ Figure out what goes in the blank: $\Big(? \Big)^{2}=\dfrac{81}{64}$ Figure out what goes in the blank: $\Big({\dfrac{9}{8}}\Big)^{2}=\dfrac{81}{64}$ So $\left(\dfrac{64}{81}\right)^{-\frac{1}{2}}=\left(\dfrac{81}{64}\right)^{\frac{1}{2}}=\dfrac{9}{8}$